This should not be a problem as DITA 1.3 is a superset
of DITA 1.2. However if for any reason, you prefer to stick to DITA 1.2
elements, please download and install the XXE add-on called "Keep
using DITA 1.2". This is done using menu item Options → Install Add-ons.

Using the DITA W3C XML Schema or RELAX NG schema rather
than the DITA DTD

By default, a DITA document created using
XMLmind XML Editor (e.g. File → New) conform to the DITA DTD and not to the W3C XML Schema or
to the RELAX NG schema.

This can be easily changed by uncommenting
out the alternative documents templates found in
XXE_install_dir/addon/config/dita/topic.xxe,
map.xxe, bookmap.xxe,
ditaval.xxe.(1). Excerpts from
XXE_install_dir/addon/config/dita/topic.xxe: